We have previously shown that extracellular ATP, like norepinephrine (NE), induces expression of the immediate-early genes (IEGs) c-fos and jun-B in cultured neonatal myocytes and that the intracellular signalling mechanism(s) activated by ATP and responsible for these changes in gene expression differ from those stimulated by NE (Zheng et al, Circ. Res., 74:1034-1041, 1994). While NE increased protein synthesis and cell size in neonatal cardiomyocytes, ATP did not. Since ATP is co-released with NE fro sympathetic nerve endings in the heart, we investigated whether ATP could modulate cardiomyocyte hypertrophy induced by NE. We found that extracellular ATP inhibited NE-stimulated increases in protein synthesis an cell size in neonatal rat cardiac myocytes. This inhibition by ATP was dose-dependent, with a Ki of approximately 25 uM. The presence of ATP throughout the period of NE stimulation was not required, since pretreatmen of the cells with ATP for as little as 1 hour prior to the addition of NE was sufficient to inhibit the increase in protein synthesis that normally occurs over the next three days in the presence of NE alone. When added after NE-induced hypertrophy was in progress, ATP inhibited any further increase in protein synthesis. ATP also affected changes in gene expressio associated with hypertrophy, decreasing both basal and NE-stimulated increases in ANF, MLC-2, and `-skeletal actin mRNA gene expression. On the other hand, ATP had no effect on changes in sarcoplasmic reticulum Ca2+ATPase (SRCA) MRNA levels caused by NE stimulation nor did it affect th level of other constitutively expressed mRNAs in the cell, such as that for glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Additionally, neither th NE-stimulated increase in IEG expression nor the NE-induced tyrosine phosphorylation and activation of the p44ERK1 and p42ERK2 mitogen activated protein kinases (MAPK) were inhibited by ATP. These results demonstrate that extracellular ATP can alter the response of cardiomyocytes to adrenergic stimulation and negatively regulate of the hypertrophic process.